The present invention relates to the manufacture of semiconductor wafers, and, in particular, relates to the properties of thin films placed thereon.
High performance Gallium Arsenide (GaAs ) and Indium Phosphide based device manufacture is increasingly being driven by epitaxial growth issues. In order to obtain good reproducibility, epilayer growth systems need periodic calibration of the growth rates (obtained as thickness divided by growth time) and of the ternary composition values. A characterization process that takes too much time, money or manpower obtaining the calibration results is not going to be used as much as the epitaxial layer grower would want.
Uniformity optimization is a second area in which the characterization process can have a significant impact. Large area epilayer uniformity is a major performance objective for epitaxial layer growth systems since it directly affects device yield. However, the epitaxial layer growers have had few choices for obtaining epilayer uniformity data in a timely fashion. Thickness and composition mapping by traditional methods are either destructive, labor intensive and/or very time consuming. Therefore, growers generally have had to rely on very limited mapping, if any, in order to try to optimize uniformity for a given process.
The present invention describes a new characterization process that measures the thickness and composition of epitaxial layers of semiconductor films. It is based on an optical characterization technique called spectroreflectance, also called optical reflectance spectroscopy. It is a technique that is widely used to measure the thickness of thin layers deposited onto a substrate. For example, spectroreflectance is used routinely in the optical coating industry to characterize the coatings deposited on lenses, mirrors, etc. After thin film deposition by a suitable deposition process, for example, chemical vapor deposition, the reflectance wavelength spectrum of the sample is then measured in a spectroreflectance apparatus. The reflectance spectrum is then numerically fitted on a computer to extract the thicknesses (growth rates) of the layer, or multiple layers, making up the deposited film.
In the spectroreflectance technique, a light beam is directed at the sample's surface and the specularly reflected light intensity is measured as a function of wavelength. The index of refraction change that occurs at each of the interfaces between the deposited layers causes a portion of the entering light to be reflected. The multiple reflected beams combine at the detector and result in constructive or destructive interference. A wavelength scan of a single layer structure having a different index of refraction than the substrate, for example, will show maxima/minima at .lambda./4n, 3.lambda./4n, 5.lambda./4n, . . . , where n is the index of refraction of the deposited film and .lambda. is the wavelength of the measuring light. For multiple layer structures, it is possible to fit the reflectance spectrum to a well-known optical model called the transfer-matrix model, also called the characteristic matrix model. The model requires that one knows the layer sequence of the deposited layers and the index of refraction as a function of wavelength of each of the deposited layers. The result of the fitting procedure, as it has been normally applied in the past for semiconductor films, is the thicknesses of each of the layers.
Spectroreflectance has been used for measuring semiconductor epitaxial layers of silicon and GaAs since the early 1960's. In this case the doping of the layers provides enough of a change in the refractive index compared to the substrate to cause the interference effects seen in the reflectance spectrum. The technique was extended to extract other Group III, Group V (III-V) semiconductor thicknesses. In these systems, the refractive index differences result from differences in the materials themselves rather than from a doping effect.
Recently, the spectroreflectance was used to wafer map the layer thicknesses precisely. The key to the high precision is the growth of a Fabry-Perot (FP) test structure. The FP structure used for thickness characterization consists of multiple pairs of Aluminum Arsenide (AlAs)/GaAs grown on a GaAs wafer and topped off with a GaAs cavity layer. The top mirror in this case is the air/GaAs interface. This type of FP structure is known to be highly sensitive to the stack and cavity thicknesses. The results of Paduano and Weyburne in J. Elec. Mat., 24, 1659 (1995) indicate that the spectroreflectivity technique can measure the variation in thickness down to better than .+-.0.1% (.about.one atomic layer) on a typical cavity thickness of .about.300 nanometers (nm).
While the spectroreflectance technique has been very successful for extracting thicknesses, relatively little work has been carried out for deducing ternary composition using reflectance spectroscopy. One group was able to use spectroreflectivity to measure the aluminum gallium arsenide (AlGaAs) composition in a vertical cavity laser structure. However, they had to start the fitting procedure using the thickness obtained by scanning electron microscope measurement and the composition obtained from a photoluminescence measurement. Similarly, others have used spectroreflectance measurements to obtain composition of a simple AlGaAs/GaAs structure but, when calculating the reflectivity for a more complicated Bragg stack, they kept the composition constant. Prior work by Tarof, et. al. was able to extract the composition for a simple one or two layer, one composition indium phosphide based ternary structure using an analytical expression that relates modulation amplitude of the reflectance to the thickness and composition of the epitaxial layers.
One of the reasons there has not been more effort in extracting the ternary composition as well as the thickness using spectroreflectance is the perceived coupling problem. In the spectroreflectance technique, it is the optical thickness given by n*d, where n is the index of refraction, which is composition dependent, and d is the actual deposited layer thickness. Thus composition and the thickness are coupled in the sense that by increasing n and decreasing d, for example, the measured optical thickness remains the same. The coupling becomes a problem since, in certain experimental conditions, n and d are essentially constants. For example, many of the early optical reflectivity measurements were done in spectrophotometers with reflectance apparatus in the 2,000-20,000 nm range. In this wavelength range, the index of refraction of GaAs and AlGaAs is slowly varying. For example, in the range 5,000 to 20,000 nm, Al.sub..3 Ga.sub..7 As average index of refraction variation is 1.6.times.10.sup.-5 /nm. The index of refraction can be experimentally determined to 3-4 decimal places. Thus both n and d are nearly constants over 1,000 nm wide wavelength ranges. This makes it possible to obtain a whole set of n's and d's that give the same optical thickness.
Thus, there exists a need for a process of measuring the thickness and composition of semiconductor thin films deposited epitaxially on semiconductor wafers.